Microfluidic devices present a cost-effective mechanism for performing small-scale fluidic manipulation on various fluid-entrained samples. For example, some microfluidic devices may be used to route, sort, and analyze cells contained in a fluid sample.
Multilayer soft lithography (MSL) is by far the most widely used approach for fabricating microfluidic devices. Numerous devices, from simple monolayer polydimethylsiloxane (PDMS) channels to multilayer structures with pneumatically controlled pumps and valves, have been used to provide versatile microfluidic functions including liquid delivery, mixing, and metering. Microfluidic large-scale integration (mLSI) has been realized in the form of microfluidic multiplexers to individually address thousands of valves and hundreds of chambers for conducting complex and multistep biochemical analyses, e.g., in lab-on-a-chip devices. Most multilayer PDMS devices demonstrated so far are not true 3D microfluidic devices. Although multiple layers of 2D microfluidic networks can be stacked, there is typically no interlayer fluid communication due to the difficulty of fabricating high-resolution through-layer vias for fluidly connecting different layers in high yield. Without through-layer vias, fluid routing and interfacing become complex issues for large scale 3D microfluidic networks.
One function that microfluidic devices may provide is cell or particulate sorting. For example, a fluid sample may have a variety of different types of cells or particles entrained within, and it may be desirable to isolate or concentrate cells or particles of a particular type with respect to the overall sample. Dielectrophoresis (DEP) is one of the most commonly used mechanisms exploited to sort cells or particulates. DEP refers to induced particle motion along an electric field gradient due to the interactions between induced electric dipoles of the particles and the applied electric field. The DEP force acting on a spherical particle, FDEP, suspended in a medium may be expressed as:{right arrow over (F)}DEP=2π∈1 Re[K(ω)]r3∇E2 where r is the radius of the particle, K is the Clausius-Mossotti factor, E is the electric field strength, ω is the angular frequency of the applied field, and ∈1 is the dielectric permittivity of the media. Since the resultant force is dependent on the electric field intensity gradient, ∇E2, the particle can be attracted towards any inhomogeneities in the field, created for example by the metallic patches on micropatterned-templates. The sign and the effective polarizability of the spherical particle may be expressed as:
                              Re          ⁡                      [            K            ]                          =                                            ɛ              2              *                        -                          ɛ              1              *                                                          ɛ              2              *                        +                          2              ⁢                                                          ⁢                              ɛ                1                *                                                                                  ɛ          1          *                =                              ɛ            1                    +                                    σ              1                                      j              ⁢                                                          ⁢              ω                                                                    ɛ          2          *                =                              ɛ            2                    +                                    σ              2                                      j              ⁢                                                          ⁢              ω                                          where σ1 is the conductivity of the media and ∈2 and σ2 dielectric permittivity and conductivity for the particles. If Re[K] is positive, particles move towards the strong electric field regions; in contrast, If Re[K] is negative, particles move to the low electric field regions.
Thus, a cell or particulate subjected to a non-uniform electric field experiences a force due to DEP effects. The magnitude of the force is dependent on various factors, including the dielectric signature of the cell or particulate, as well as the frequency of the electric field. Depending on the DEP field used and the characteristics of the individual cells or particulates subject to the DEP field, cells or particulates may experience either positive DEP (experiencing force that urges the cell or particulate in the direction of increasing field strength) or negative DEP (experiencing force that urges the cell or particulate in a direction opposite of increasing field strength). In many cases, the movement of cells or particulates via DEP may be practically limited to approximately 100 μm/s given the characteristics of those cells or particulates, the media that are commonly used to transport them, and the electrical characteristics of microfluidic systems.
DEP response of cells or particulates may be altered or enhanced by tagging cells or particulates of interest with molecules, e.g., labeled or unlabeled antibodies, or beads that are specific to certain cells or particulates of interest. This may allow for easier separation of the target cells or particulates using DEP. While such tagging can enhance DEP techniques, it is not necessary in many cases.
FIG. 1 depicts one example of a two-dimensional DEP cell sorter 100. FIG. 1 shows only a plan view of a portion of a PDMS layer of the cell sorter 100; in actual practice, the PDMS layer would be sandwiched between two plates, e.g., glass plates, that are not shown. The PDMS layer may include a sample channel 102 and a buffer channel 104 that run parallel to one another and that are separated from one another by a thin wall 116. A fluid sample and a buffer may be flowed into the sorter from, with respect to the orientation of FIG. 1, left to right via their respective channels. The thin wall 116 may have an opening 118 that permits fluid communication between the sample channel 102 and the buffer channel 104. Patterned electrodes 106 may extend at an angle across the sample channel 102; a patterned electrode 106 may be patterned on each of the two plates. An electromagnetic field may be produced within the sample fluid that is flowed through the sample channel 102 between patterned electrodes 106 when an alternating current is used to produce a voltage across the electrodes. Depending on the frequency of the electromagnetic field, certain cells, e.g., “square” cells 110, may be drawn towards the maximum field strength and “round” cells 108 may be repulsed or unaffected. The angled nature of the electromagnetic field (due to the angle of the patterned electrodes 106) may cause the square cells 110 to be drawn towards the buffer channel 104 as the fluid flow in the sample channel 102 and the buffer channel 104 progresses, with respect to the orientation of FIG. 1, from left to right. After the end of the opening 118, the square cells 110 that have been shunted towards the buffer channel 104 may flow into a collection channel 112, whereas the round cells may flow into a waste channel 114.
FIG. 2 depicts another example of a two-dimensional DEP cell sorter 200. In this case, the cell sorter 200 includes two sample channels 202 that bracket a buffer channel 104. A fluid sample may be flowed into the cell sorter 200 via the sample channels 202 while a neutral buffer may be flowed into the cell sorter 200 via the buffer channel 204. The combined buffer/sample fluid flows through a sorting region containing a number of patterned electrodes 206. When the patterned electrodes 206 are powered at a particular frequency, the resulting electromagnetic field may cause the square cells 210 to migrate towards the center of the cell sorter 200, whereas the round cells 208 may migrate towards, or stay in, the outer edges of the cell sorter 200. The center-concentrated square cells 210 may then flow into a collection channel 212, whereas the round cells 208 may flow into waste channels 214.
Two-dimensional cell sorters typically have a maximum flow rate beyond which the cell sorting functionality is lost or significantly impaired. The forces produced by DEP, and consequently the rate at which DEP can move cells across the flow stream and into position for flow into the collection channels, are limited by the size and shape of the electrodes as well as other system characteristics. If the fluid flow rate is fast enough that the cells flow past the patterned electrodes before the forces produced by the DEP effect can re-position the cells for flow into the collection channels, then the cells will not be effectively sorted. This limits the maximum flow of two-dimensional cell sorters, and, consequently, the maximum throughput of a two-dimensional cell sorter. Such two-dimensional DEP cell sorters are thus typically limited to maximum flow rates of approximately 1 mm/sec, which, in turn, limits the throughput of such cell sorters.